(1) Field of the Invention
The present invention relates to a thermal transfer image recording sheet which will be referred to as an image recording sheet hereinafter. More particularly, the present invention relates to an image recording sheet having an enhanced resistance to curling.
(2) Description of the Related Art
It is known that the thermal transfer image recording systems are classified into thermal sublimating dye transfer systems in which a thermal sublimating dye is carried on a support sheet and is used in combination with a sublimating dye image-receiving sheet having a dye-receiving resin layer for recording dye images thereon, and dye images are transferred from the thermal transfer sheet to the sublimating dye-receiving sheet upon heating; and thermal ink melt transfer systems in which a thermal transfer sheet in which an ink comprising a thermally fusible substance, for example, a paraffin wax and a coloring pigment or dye is carried on a support sheet and is used in combination with an ink image-receiving sheet having an ink-receiving pigment and/or resin-containing layer, and the ink melt is transferred imagewise from the thermal transfer sheet to the ink receiving layer by heat-melting the ink. Both of the thermal transfer systems can record full colored images on the image receiving sheet.
Particularly, the thermal sublimating dye transfer system can produce clear full colored prints and has thus attracted public attention.
The thermal transfer sheet includes yellow, magenta and cyan-coloring dye sheets and optionally a black-coloring dye sheet. The full colored images are formed by arranging that each thermal transfer sheet is superposed on a dye image-receiving sheet and is heated imagewise by a thermal head attached to a printer so as to transfer the dye images from each thermal transfer sheet onto the image-receiving sheet. The transferred dye images are placed on other dye images so as to form the desired colored images.
In the case of the thermal sublimating dye transfer system, to obtain printed images with good quality, usually, an image-receiving sheet comprising a substrate sheet selected from monoaxially or biaxially orientated plastic resin films and multi-layered plastic resin films (namely, synthetic paper sheets), and an image-receiving layer formed on the substrate sheet and comprising, as a principal component, a sublimating dye-dyeable resin, is used. The above-mentioned dye image-receiving sheet is advantageous in that the thickness is even, the softness is satisfactory, the heat-conductivity is lower than that of conventional cellulose fiber paper sheets, and thus the transferred dye images having high color density and uniformity can be obtained.
In the production of the plastic film or the synthetic paper sheet, the films are oriented in one or two directions, and a residual orientation stress remains therein. Thus, when an image-receiving sheet comprising a plastic film and/or a synthetic paper sheet is subjected to a thermal transfer procedure, the residual stress in the plastic film and/or the synthetic paper sheet is released by heating so as to cause the plastic film and/or the synthetic paper sheet to shrink and thus curls or wrinkles are generated on the image-receiving sheet. The curls and wrinkles in the image-receiving sheet make it difficult to pass through the printer and the commercial value of the resultant prints significantly decreases.
To solve the problems derived from the materials for the substrate sheet, it has been attempted to laminate the above-mentioned monoaxially or biaxially oriented plastic films or synthetic paper sheets, on front and back surfaces of a core sheet, for example, a paper sheet, having a low thermal shrinkage or elongation, so as to balance the tensions created on the front and back surfaces of the resultant substrate sheet and to control the curling and/or wrinkling.
However, when the above-mentioned multi-layered substrate sheet is coated with an image-receiving layer to produce a thermal transfer image recording sheet, the plastic films or synthetic paper sheets in the substrate sheets thermally shrink and thus the resultant thermal transfer image recording sheet is curled or wrinkled and becomes commercially worthless. Namely, when a multi-layered substrate sheet is produced, the element sheets or films are usually bonded to each other through a polymeric binder by a dry laminating method. The polymeric binder for the dry lamination usually comprises a binder component selected from polyether and polyester binder compounds and a curing component selected from isocyanate and epoxy curing compounds and mixed with the binder component. The conventional polymeric binder is preferably selected from those having a glass transition temperature higher than 50.degree. C., to enhance the bonding strength between the bonded element films or sheets. When the conventional multi-layered sheet is heated so as to cause element films or sheets to shrink at different rates from each other, the conventional cross-linked polymeric binder layer does not shrink and thus cannot absorb the shrinking energy of the element films or sheets. Accordingly, the conventional multi-layered sheet curls.
In view of the above-mentioned problems, there is a strong demand of a thermal transfer image recording sheet having an enhanced resistance to curling.